The long-range goal of my research program is to determine the perceptual, cognitive, and social factors that underlie social perception, particularly focusing on face perception. The proposed research will investigate the basic of categorization by macaque monkeys. Monkeys will be trained to perform a classification (categorization) task in which they are rewarded for responding discriminatively to photographs containing human faces or monkeys faces. The basis for differential responding to photographs of natural categories has been debates in the literature. One explanation has been that the categories are predetermined by the evolutionary history of the animal, and that the basis for the response is conceptual. The opposing explanation has been that the animals performing this task respond on the basis of individual perceptual features of the photographs, classifying individual exemplars on the basis of physical similarity. The specific aims of the proposed research are (1) To determine whether physical similarity judgements of particular stimuli affect the accuracy and latency of classifying the stimuli into human and monkey categories, (2) To determine whether degradation of the stimuli which eliminates information about physical identify affects the monkey's ability to make category assignments, and (3) To determine whether achromatic photographs and line-drawing representations of social stimuli are classified as accurately and quickly as color representations. Taken together, the obtained results of a series of studies designed to address these specific questions will provide data relevant to the theoretical question concerning the basis for discriminative responding to photographic exemplars of face stimuli and will have implications for natural concept learning. Defining the basis for categorization of faces by monkeys, as proposed here, will provide a strong empirical foundation for future studies of the neural basis of face categorization, identification, and recognition in macaque monkeys and should lead to a productive animal model of human visual agnosia.